The present invention relates to fluid coupling devices of the type including both fluid operating chamber and a fluid reservoir chamber, and valving which controls the quantity of fluid in the operating chamber.
Although the present invention may be used advantageously in fluid coupling devices having various configurations and applications, it is especially advantageous in a coupling device of the type used to drive a radiator cooling fan of an internal combustion engine, and will be described in connection therewith.
Fluid coupling devices (“fan drives”) of the viscous shear type have been popular for many years for driving engine cooling fans, primarily because their use results in substantial saving of engine horsepower. The typical fluid coupling device operates in the engaged, relatively higher speed condition only when cooling is needed, and operates in a disengaged, relatively lower speed condition when little or no cooling is required. Today, electrically actuated viscous fan drives are commonplace because they can be precisely controlled between an engaged, partially engaged, and disengaged mode to control output at a given fan speed as determined by the vehicle's engine computer.
Today's electrically actuated viscous fan drives have the actuator mounted to either the front or the rear side of the fan drive. In both cases, the actuators are mounted to the drives through a ball bearing and the stationary electrical wires are then tethered to a stationary location on the engine or shroud or whatever optimum for the particular customer application. The length of tether for front mount actuators becomes a limiting factor for large fan applications and the axial length of the rear mount actuator limits the use from narrow package applications. Durability of either design is a function of bearing life and tether life. Ideally, a fan drive without a tether would be preferred if this improves durability and lowers cost while sustaining fan drive performance attributes.
The front mounted electrical actuator was result of an evolution of earlier air-actuated viscous fan drives used in heavy truck and large bus applications. The bi-metal control spring on the front of the viscous drive was simply replaced by a bearing mounted pneumatic solenoid. Durability issues with the tether and higher fuel economy requirements forced the heavy-duty industry to switch to pneumatic on-off friction clutches with no tether (air supply coming through the center of the mounting bracket-pulley subassembly). Today the heavy-duty industry is now facing even stiffer fuel economy and noise control requirements which has forced a need for variable speed or at least multi-speed fan drives. As a result, viscous drives are being considered again which has lead to the need for rear-actuated viscous fan drive. Subsequently, a rear mount electrical actuator was developed which has helped reduce potential tether durability problems associated with the front mount style actuator and in addition provides the customer an easier means to install the fan drive and associated tether.
Front actuated viscous fan drives continue to exist though for light to medium duty applications because the axial length and cost are better than rear actuated. However, in some light duty gas engine applications where the fan clutch is driven by the waterpump, a system resonance problem exists caused by numerous factors including mass and cg of the fan drive.